It is well known that clock-driven CMOS logic can be kept in a low power state if its main driving clock is shut off. It is also known that the power consumed by such Complementary Metal Oxide Semiconductor (CMOS) logic or the like tends to increase as its clocking frequency increases. On the other hand, it is understood that the speed at which clock-driven sequential CMOS logic or the like can respond to emergency situations tends to slow as its clocking frequency is slowed. Thus, in situations where it is desirable to have both low power consumption and fast, logical response by a sequential logic means to exigent circumstances, these two characteristics of CMOS logic (or its equivalents) conflict with one another.
A nonlimiting example of such a situation may be found in the art of battery powered systems and the like (i.e., systems driven by low power photovoltaic panels). These are sometimes operated in a low power or sleep mode for extended periods of time in order to conserve on power draw when not needed, so as to thus extend the useful duration of charge storage within a battery system or another charge storing system. In the sleep mode, part or essentially all of system activity is disabled. Some portions of the system (e.g., volatile CMOS memory) may need to be maintained above a predetermined minimum sleep voltage, VSLEEPmin, so that vital operational data stored therein is not lost and/or so that the system may remain sufficiently alive to be able to respond to new inputs as they sporadically present themselves, especially those that threaten continued viability of the system.
In some situations it is desirable for the mostly-asleep system to be able to quickly wake up in part or in whole and to quickly respond to newly emerging situations as appropriate. One situation that may call for rapid response is imminent loss of the minimum sleep voltage, VSLEEPmin, as applied to essential memory; this for example being caused by parasitic loss of charge in the system-preserving, minimal power supply means due to passage to time and/or other causes (i.e., temperature change). In some systems, a voltage slightly above the minimum sleep voltage, VSLEEPmin is stored within a low-leakage capacitor (a system-preserving, minimal power supply means) with the main chemical or other battery being effectively disconnected from major loads so as to better extend battery life. Eventually, charge leaks out from even the low-leakage capacitor and it becomes necessary to temporarily recouple the main battery directly or indirectly (i.e., through a switched regulator) to the minimized load and capacitor so as to recharge the capacitor (or other system-preserving, minimal power supply means) back to VSLEEPmin or above (e.g., to a predetermined, deadband high voltage, Vdbh such as disclosed in above cited, U.S. Ser. No. 11/030,688). In one subset of such systems, the low-leakage capacitor is recharged by application of a single recharge pulse to slightly above VSLEEPmin (e.g., to 10% above) and then allowed to decay back down to VSLEEPmin before the next recharge is applied. Other means for recharging the discharged capacitor may be used besides effective reconnection of the main battery, including for example, temporary connection to a power grid or to a photovoltaic panel. The battery is just an illustrative example.
One problem with such mostly-asleep systems is that upon awakening, the system may not have adequate knowledge of the state of its surroundings and that state could be rapidly deteriorating or otherwise changing. So a rapid attaining of situational awareness may be needed and a rapid response thereto may also be needed. By way of example, if one or both of a combination of battery and photovoltaic power is to be used for recharging a charge-maintaining capacitor during an arbitrary awakening time, the system may need to intelligently decide which source to use and also what duration of recharge pulse might be needed. That could depend on whether awakening occurs during night or day, and if the latter, whether it is sunny or cloudy at the time. Duration of the recharge pulse may also depend on the state of the main battery, which itself may have been subject to voltage decay due to passage of time and/or due to temperature change. The awakening system should be able to quickly assess its situation and make an intelligent decision of what to do next. But how can it do so if its main clock is turned off to save energy? On the other hand, if the main drive clock is left always on, how can the mostly asleep system keep its power draw low while asleep?
From the above, it can be seen that there is need for a low power means of maintaining situational awareness. There is need for a monitoring means that can detect changed condition such as when a voltage across a sleep mode voltage source (e.g., the low-leakage capacitor) is dropping perilously close to (or below) a predefined minimum sleep voltage, VSLEEPmin. The monitoring means should be able to quickly indicate that corrective action needs to be taken once an alert-worthy condition is detected. In order to preserve power, however, the monitoring means and its method of monitoring should themselves be relatively low powered ones. On the other hand, in order to provide quick alert when an alert-worthy condition is detected, the monitoring means and its method of alerting should be of relatively high speed.
From the above, it can be further seen that, for a sleeping and exigently-awakened system, it is desirable to not only have a low power monitoring means but to also have an intelligent response means (e.g., logic means) that is rapidly and properly clocked so it can respond quickly and correctly to each alerted situation. Conventional practice uses PLL's (phase locked loops) for precision control of the main clock drives of microprocessors and like controller logic. Such PLL's generally have slow reaction time and may take on the order of many milliseconds to lock onto a desired clocking frequency in view of external temperature and/or voltage conditions. Such start up delays may be unacceptable in emergency wake-up situations such as described above.